There are a variety of types of footwear such as men's leather shoes, women's leather shoes, sport shoes, campus shoes, women's chemical shoes, men's chemical shoes, Hepburn sandals, medical shoes, and safety shoes. Natural and synthetic materials such as leather, rubber, polybutadiene materials, polyvinyl chloride materials, polyurethane materials, and polyester materials are widely used as raw materials for shoe soles of such footwear.
Among these materials, polyurethane foam (cellular polyurethane elastomer) surpasses other materials in terms of lightweightness, mechanical strength, wear resistance, bending resistance, chemical resistance, anti-slip property, cold-resistance, moldability, etc., and is thus used in a wide variety of applications.
However, recently, the shoe sole market inside and outside Japan is maturing rapidly. In the footwear industry, various attempts have been actively made to increase the added value, such as imparting various properties to footwear to improve foot comfort and enhancing ecological benefits (environmental conservation measures), e.g., reduction in CO2 emission when products are discarded and incinerated and facilitating recycling of the products.
Vast quantities of petroleum-derived materials are presently used as the main material of polyurethane foam resin compositions for shoe sole applications. Since there is a concern that this may pose a problem in terms of environmental conservation in the future, measures have to be taken to avoid the problem.
Meanwhile, the problem of future depletion of petroleum resources is gathering much attention. A trend toward relying more on biomass materials such as plant-derived materials and less on conventional products that use petroleum-derived materials is quickly emerging. In the polyurethane foam industry also, use of biomass materials such as polyol components extracted or purified from castor oil and soy beans is highly anticipated.
Recently, regarding the use of biomass materials produced from biomass raw materials, Japan BioPlastics Association (JBPA) has established an identification system for biomass plastics which are biomass plastic products containing a particular amount or more of organic source (plants etc.)—derived substances as a plastic component in order to make consumers take due account of the importance of preventing global warming and reducing the consumption of petroleum resources. Under this system, products that comply with the standards set by the association are certified as “Biomass Pla” and are authorized to use a logo so that general consumers can easily identify the biomass plastic products.
In order to obtain authorization for biomass pla identification, the “biomass plastic ratio” of the product (the ratio of the biomass-derived components in the product relative to the total amount of the product) needs to be 25% by mass or more. Increasing the biomass plastic ratio in the product (at least 25% by mass) is very important for reducing the impact of the products on the environment. Obtaining a biomass pla certification mark will become essential from the viewpoint of increasing the awareness of the need to reduce the environmental impact for reducing environmental impact for both manufacturers and consumers.
In order to reduce the impact on the environment, various proposals have been made regarding two-part liquid polyurethane foam resin compositions that use a plant-derived material, namely, a castor oil polyol, instead of conventional petroleum-derived raw materials, and regarding molded articles made using such compositions.
A known example of a polyurethane foam made of a polyurethane raw material containing a polyol, a polyisocyanate, a blowing agent, and a catalyst is a soft polyurethane foam that uses both a polyol having two to four functional groups and being composed of a fatty acid and an ester condensate of a polyfunctional glycol, and a polyol which is a polyether polyol having a polyoxyalkylene chain and which contains 45% to 85% of ethylene oxide in the polyoxyalkylene chain, in which at least toluene diisocyanate is used as the polyisocyanate. This soft polyurethane foam uses a castor oil polyol as the polyol having two to four functional groups and is reported to exhibit low hardness, low strain, and good breathability (for example, refer to PTL 1).
However, the soft polyurethane foam described in PTL 1 has a low hardness and insufficient strength and has a problem in that shoe soles made from the soft polyurethane are lacking in comfort. There is another problem in that the strength is insufficient for use in industrial components such as packings, hoses, sheets, and cushioning materials, decoration cushioning components such as furniture and mattresses, packaging components, and vehicle components such as vehicle bumpers.
An example of a polyurethane foam obtained by mechanical stirring foaming of a polyurethane raw material containing a polyol component and an isocyanate component is a polyurethane foam that contains a castor-oil-modified polyol having a hydroxyl value of 80 to 360 mg/KOH and an average functional group number of 2.5 to 6 as a polyol component and at least one selected from a modified tolylene diisocyanate (modified TDI), a modified diphenyl methane diisocyanate (modified MDI), and modified hexamethylene diisocyanate (modified HDI) having a NCO content of 1 to 15% as the isocyanate component. This polyurethane foam has low hardness and low permanent set and a conductive roller using such a foam in an elastic layer has been reported to prevent occurrence of contact marks, effectively preventing the occurrence of image defects caused by the contact marks, and reliably form high-quality images (refer to PTL 2).
However, since the average functional group number of the castor oil polyol used in the polyurethane foam described in PTL 2 is as large as 2.5 to 6 and the flexibility is thus low, there has been a problem in that cracking occurs in an article, such as a shoe sole, which is repeatedly subjected to bending.
A two-part liquid cellular polyurethane foam elastomer composition for shoe soles, the composition containing an organic polyisocyanate and a polyol prepared by addition polymerization of lactone to polyoxytetramethylene glycol is also known. This two-part liquid cellular polyurethane elastomer composition for shoe soles has mechanical strength (tensile strength and tear strength) and water resistance and exhibits good bending resistance, wear resistance, and mold resistance. It is thus reported that this composition is suitable for use in soles of sport shoes such as jogging and tennis shoes, high-grade men's leather shoes, and safety shoes (for example, refer to PTL 3).
However, the two-part liquid cellular polyurethane foam elastomer composition for use in shoe soles described in PTL 3 uses petroleum-derived raw materials and has a high impact on the environment, possibly posing a problem regarding environmental conservation.
As discussed above, in order to reduce impact on the environment, there has been a high anticipation on development of a two-part liquid polyurethane resin composition that uses little or no conventional petroleum-derived raw materials but uses a plant-derived raw material to improve the biomass ratio and reduce the environmental impact while exhibiting excellent physical properties (strength, elongation, flexibility, dimensional stability, and wear resistance), a molded article using the composition, and a shoe sole.